Electromagnetic Spectrum

Key words: photons, electrons, magnetic field, waves,  electromagnetic waves, light, radio waves, infra-red, visible spectrum, ultra violet, microwaves, gamma rays, x-rays.

To start with you should know what a spectrum is: when white light is shone through a prism it is separated out into all the colours of the rainbow; this is the visible spectrum. So white light is a mixture of all colours. Black is NOT a colour; it is what you get when all the light is taken away.

That was easy. Some physicists pretend that light consists of tiny particles which they call photons. They travel at the speed of light (what a surprise). The speed of light is about 300,000,000 meters per second. When they hit something they might bounce off, go right through or get absorbed. What happens depends a bit on how much energy they have. If they bounce off something and then go into your eye you will “see” the thing they have bounced off. Some things like glass and perspex will let them go through; these materials are transparent. Black objects absorb the photons so you should not be able to see black things: you will have to think about this one. These poor old physicists get a little bit confused when they try to explain why some photons go through a leaf, some are reflected, and some are absorbed. They say that it is because they have different amounts of energy.

Other physicists pretend that light is made of waves. These physicists measure the length of the waves and this helps them to explain what happens when light hits leaves. The light with the longest wavelength (red) is absorbed by the green stuff (chlorophyll) in the leaves. So is the light with the shortest wavelength (blue). In between these two colours there is green light, this is allowed to pass right through or is reflected. (Indigo and violet have shorter wavelengths than blue light.)

Well it is easy to explain some of the properties of light by pretending that it is made of tiny particles called photons and it is easy to explain other properties of light by pretending that it is some kind of wave.

This page is all about electromagnetic waves. The visible spectrum is just one small part of the electromagnetic spectrum. These electromagnetic waves are made up of two parts. The first part is an electric field. The second part is a magnetic field. So that is why they are called electromagnetic waves. The two fields are at right angles to each other.

Radio Waves
Infra Red
The Visible Spectrum
Ultra Violet
Gamma Rays

Radio Waves:

I like these waves; they are used for radio broadcasts, amateur radio, television, and mobile phones. Different parts of the radio spectrum have been allocated to the various services. Radio waves have a much longer wavelength that light waves. The longest waves are several kilometers in length. The shortest ones are only millimeters long.

(more to come when I have the time.) Here is a bit more…..

The really nice thing about radio waves is that they will make the electrons in a piece of copper wire move; this means that they generate electric currents in the wire. In fact it works both ways: alternating currents in a copper wire generate electromagnetic waves, and electromagnetic waves generate alternating currents. The electric currents at “radio frequencies” (rf) are used by radio and television transmitters and receivers. I use frequencies of 145MHz (VHF), 14MHz (20m HF) and 3.7MHz (80m HF).

My HF radio transceiver can operate on all the HF amateur bands, but 20m and 80m are my favorites at present. On the 80m band the transceiver produces rf between 3.5MHz and 3.9MHz, the part of the transceiver which does this is called an oscillator. I usually start with 3.77MHz but I can change the frequency with the tuning knob. When I transmit, the transceiver “modulates” the rf with an alternating current generated by my voice in a microphone. The modulating frequency is called “af” or audio frequency. Someone listening to me uses a receiver which can “demodulate” the radio signal. This means that Microwaves:


Microwaves have such a short wavelength that they are very easily absorbed by water. This is why they are used in microwave ovens. What happens is that when the water in your TV dinner absorbs the microwaves, the energy of the microwaves is converted into heat: it makes the water molecules vibrate faster. Some people are frightened that the radio waves coming out of their mobile phones are short enough to cook their brains.

Infra Red:

These radio/light waves have a very short wavelength; their wavelength is longer than visible light. Infra-red can be detected by special infra-red film. If the police or MI5 shine an infra-red light on you they will be able to take a picture of you using infra-red film: you will not know that they have taken your photo. You have been warned!!! Animals like the pit-viper have infra-red detectors so that they can find their prey in the dark. You have been warned again!!!

The Visible Spectrum:

Richard Of York Gave Battle In Vain. = Red, Orange, Yellow, Green, Blue, Indigo, and Violet. are the colours of the visible spectrum. We cannot see Infra-red, but we can feel it warm our skin when we sit in the sun. Infra-red has a longer wavelength (less energy) than Red light. We cannot see Ultra-violet light, but we feel our skin has been burnt by the sun if we were in the sun too long yesterday. It is the Ultra-violet which is thought to cause skin cancer. UV light has a shorter wavelength (more energy) than visible light.

Ultra Violet:

These waves have very high energy and very short wave lengths; shorter than visible light. Some animals like honey bees can see ultra-violet light. Some plants have white flowers, at least you think that they are all white, but they may appear to be different colours to a honey bee because of the amounts of ultra-violet light which they reflect.


X-Rays have so much energy and such a short wavelength that they can go right through you. However, they cannot get through bone as easily as they can get through muscle. This is because your bones contain so much Calcium. If you have never had an X-Ray, try jumping off a high wall and breaking a bone. The doctors will soon have you fixed: they take an X-Ray to see which bones have been broken. If you break the bone in enough places they will have to use steel bolts to fix you up.

X-Ray can also be used to find other problems in your body. If the doctors want to look for ulcers in your guts, they can give you a Barium meal. Like Calcium, the Barium absorbs X-Rays so the doctors can look at parts of your guts and find your ulcers. You get ulcers by

Gamma Rays:

These are nasty ones. They have very high energy and will even go through metals. So they can be used for finding tiny cracks in metals. You cannot see the hairline cracks in an aeroplane wing with the naked eye. How would you like the wings to fall off the plane on your summer holiday flight to Neasden or wherever it is that you go. If the plane has been thoroughly checked you should be safe. Some radioactive materials produce gamma rays. Gamma rays and X-Rays can cause cancer, but gamma rays can also be used to destroy cancer cells: this is radio-therapy.